Cleanup: Constraints, update documentation of some flags

Update the documentation of `CONSTRAINT_DISABLE` and `CONSTRAINT_OFF`
flags to clearly distinguish them from each other.

- `CONSTRAINT_OFF`: The eye icon in the interface. Managed by the user,
  maybe indirectly via the animation system or drivers. Maps to both the
  `mute` (positive) and `enabled` (negative) properties in RNA.
- `CONSTRAINT_DISABLE`: The red color in the interface that indicates
  invalid settings. Set by Blender when a constraint is considered to be
  invalid, for example a 'Copy Location' constraint without a target.
  Maps to `is_valid` (negative) in RNA.

No functional changes.

source/blender/makesdna/DNA_constraint_types.h

@@ -672,7 +672,7 @@ typedef enum eBConstraint_Flags {
   /* Expansion for old box constraint layouts. Just for versioning. */
   CONSTRAINT_EXPAND_DEPRECATED = (1 << 0),
 #endif
-  /* pre-check for illegal object name or bone name */
+  /* Constraint is disabled because it is considered invalid. `is_valid` in RNA. */
   CONSTRAINT_DISABLE = (1 << 2),
   /* to indicate which Ipo should be shown, maybe for 3d access later too */
   CONSTRAINT_ACTIVE = (1 << 4),
@@ -680,7 +680,7 @@ typedef enum eBConstraint_Flags {
   CONSTRAINT_SPACEONCE = (1 << 6),
   /* influence ipo is on constraint itself, not in action channel */
   CONSTRAINT_OWN_IPO = (1 << 7),
-  /* indicates that constraint is temporarily disabled (only used in GE) */
+  /* Constraint is disabled by the user or the animation system (eye icon in the interface). */
   CONSTRAINT_OFF = (1 << 9),
   /* use bbone curve shape when calculating headtail values (also used by dependency graph!) */
   CONSTRAINT_BBONE_SHAPE = (1 << 10),